1. Field of the Invention
This invention relates to a process for preparing microcapsules containing a hydrophobic liquid.
2. Description of the Prior Art
Microcapsules have already found application in the production of pressure-sensitive copying papers, and a great expectation of the development of their utility in pharmaceuticals, agricultural chemicals, perfumes, and adhesives is expected. Many methods are known for preparing microcapsules, and of these, the following methods appear to be comparatively relevant to the present invention.
A. A method which comprises dissolving a water-soluble aminoaldehyde condensate in a continuous phase, polymerizing the aminoaldehyde condensate using an acid catalyst, depositing the polymer around the particles to be occluded therein which are dispersed in the continuous phase, thereby to encapsulate the particles (as disclosed in U.S. Pat. No. 3,516,941).
B. A method which comprises incorporating substances which react with each other to form a polymer which is insoluble both in a hydrophilic liquid and in a hydrophobic liquid (not including substances which self-polymerize) in each of these liquids, emulsifying one of the liquids in the other, and encapsulating the liquids by the polymerization at the interface of the hydrophilic liquid with the hydrophobic liquid (an interfacial polymerization method; as disclosed in British Pat. Nos. 1,091,076 and 1,091,077).
C. A method which comprises dissolving a substance containing a double bond in a hydrophobic liquid, emulsifying the solution in water, and polymerizing the substance containing a double bond in the droplets of the hydrophobic liquid to encapsulate the liquid (an internal polymerization method; as disclosed in U.S. Pat. No. 2,969,330).
In method (a) mentioned above, the polymer formed by polymerization in the continuous phase does not deposit only around the particles to be occluded, but particles consisting only of the polymer are formed in the continuous phase. It is difficult therefore to deposit all of the wall-forming substance used around the particles to be occluded with good efficiency. As a result, microcapsules obtained by method (a) have a very thin capsule wall, and in some cases, the particles to be occluded per se without any capsule walls (that is, unencapsulated oil droplets) remain. Accordingly, method (a) does not provide capsules having a high ability to retain the contents. Furthermore, the capsules obtained by method (a) have the defect of low mechanical strength.
According to method (b), once a thin capsule wall has been formed in the interface, the reaction between the reactant in the continuous phase and the reactant in the liquid droplets to be occluded stops. Accordingly, method (b) has the defect that the capsule wall obtained is thin, and unreacted reactants remain in the system. Furthermore, according to method (b), large quantities of reactants must be dissolved in the liquids to be occluded, and therefore, those liquids that can be occluded are limited to those which have a high solubilizing power for the reactants, and which are inert to the reactants. Also, because of the thin capsule walls obtained, a defect exists in that the ability to retain the contents in the capsules is poor, and the mechanical strength is low.